I agree with Raziel. Just leave the settings at default and it will work as well as it can.

On an XE/µA1, you will be limited by hardware issues (memory access, bus speeds and the like). Only a modern machine will let you max out the R/W speeds of an SSD.

Don't forget to leave some space on the "disk" unused, so that the device firmware can take advantage of it for wear-levelling.Don't EVER perform a full format on (even part of) an SSD, as you will mark all the unused parts of the disk as "used" and make the firmware's job that much harder.

MaxTransfer is not a rate, it is a size! It has nothing to do with speed. It only specifies the chunk size given from the file system to the device driver. Chunks bigger than 128k aren't possible with ATA anyway, so if you set it to 0x20000 or higher makes no difference.

Properly written device drivers do the split into supported chunks automatically, so MaxTransfer should always be set to 0xffffffff. Only if your driver is buggy you might have to adjust the value. It's a compatibility setting after all.

You could try to play around with the UDMA settings in UBoot. This is the only way to change transfer speed.

1st a confession, I had completely forgotten that there is a UBoot prefs program, consequently, when I posted my query, I thought it was some uboot command line command I had to run at Boot time. - Sorry for the grief, It just suddenly popped into my brain that there was a uboot prefs program. I guess at my age, my neurons took a looooong detour.

Anyhow, for the record, a MicroA1 will support a crucial 256GB SSD upto UDMA1 and no more beyond.